Abstract Ticks are the most important disease vector in the United States and Europe. Ixodes scapularis ticks can transmit at least seven pathogens relevant to human health. Why I. scapularis ticks are permissive to such a wide range of pathogens is not known. The arthropod immune system can influence its competence for pathogen transmission, but the molecular details of tick immunity remain vague. We recently identified a noncanonical Immune Deficiency (IMD) pathway in ticks that limits colonization of two bacterial pathogens: Borrelia burgdorferi (Lyme disease) and Anaplasma phagocytophilum (Human Granulocytic Anaplasmosis). The molecular and cellular events preceding activation of the noncanonical IMD pathway are currently unknown. An attractive candidate for an upstream activation mechanism is the unfolded protein response (UPR), a highly conserved cellular reaction to endoplasmic reticulum (ER) stress. ER stress can be triggered by pathogens and Anaplasma spp. have been observed to closely associate with the ER in tick cells during intracellular infection. Furthermore, the mammalian analogue to the IMD pathway, the Tumor Necrosis Factor Receptor (TNFR) network, crosstalks with the UPR and propagates innate immune responses. These observations led us to look for evidence of UPR activation in I. scapularis. We found 14 UPR genes that are differentially induced in A. phagocytophilum-infected ticks. Based on this preliminary data, our central hypothesis is that the tick UPR is activated by infection and orchestrates an antimicrobial response by crosstalking with the IMD pathway. AIM 1 of this proposal will characterize which UPR pathways are induced during A. phagocytophilum and B. burgdorferi infection of tick cells with protein biochemistry and immunofluorescent microscopy. AIM 2 will investigate whether tick UPR pathways limit A. phagocytophilum in vitro, by using transcriptional knockdown and pharmacological inhibition. AIM 3 will functionally evaluate in vivo whether tick-transmitted pathogens are limited by the UPR. To accomplish this, we will use transcriptional knockdown in ticks followed by A. phagocytophilum or B. burgdorferi challenge. The outcome of this proposal will provide insight into the molecular mechanisms that govern vector competence and immunity in ticks.